The present invention relates to a wrapped slide bearing bushing made of a composite slide bearing material with a metallic support layer, a porous bearer layer sintered on the latter, and a slide layer on a PTFE (polytetrafluoroethylene) base, which constitutes the slide surface for a slide partner, which fills pores in the porous bearer layer, and which comprises at least 60% by volume of PTFE, 15 to 25% by volume of a metallic filler material, preferably lead, and, if desired, 8 to 12% by volume of PVDF (polyvinylidenefluoride).
In the slide layer on the PTFE basis, the PTFE is intended to exert a matrix-forming function; thus, it must be present in such a percentage of the volume that it can enclose the metallic filler and the possibly also present PVDF. If no PVDF is provided, the PTFE portion should be at least 70% by volume.
The metallic filler material preferably is lead; however, it would also be possible to use zinc sulfide, for example.
The metallic support layer is preferably composed of steel, however, a highly-stable aluminum alloy or bronze could also be employed. The porous bearer layer sintered thereon preferably consists of tin bronze or leaded tin bronze.
Slide bearing bushings of this type, as well as the composite slide bearing materials on which they are based, are known. For example, Applicant offers slide bearing bushings, whose slide layer is constituted by a mixture of 80% by volume of PTFE and 20% by volume of lead. Moreover, a slide bearing material has been proposed in European Patent EP 0 632 208 A1, whose slide layer comprises 70% by volume of PTFE and 20% by volume of a metallic filler material, as well as 10% by volume of PVDF.
Wrapped slide bearing bushings made of the above described composite slide bearing materials do perform satisfactorily. They have a low coefficient of friction or frictional index and are distinguished by great wear resistance.
In connection with a special application of wrapped slide bearing bushings in shock absorbers of motor vehicles, the number of constantly occurring load changes in the course of the useful life of the motor vehicle is extremely high. Because of this, the actual slide layer, i.e. the amount projecting past the porous bearer layer, wears off, so that the individual plateaus of the porous bearer layer start to wear. The slide bearing bushing does not lose its ability to function because of this. However the damping behavior changes and the coefficient of friction is increased.
Based on the foregoing, the object of the present invention lies in providing a wrapped slide bearing bushings having an even higher wear resistance, wherein the coefficient of friction remains below 0.1 over an extended period of time.
In a wrapped slide bearing bushings of the type described at the outset, this object is attained by the present invention in that the slide layer contains 1 to 3% by volume of carbon fibers, and that the carbon fibers are essentially oriented in a preferred, or given direction with respect to the composite slide bearing material, and the relative movement of the bushings and the sliding partner used with it essentially extends transversely with regard to this preferred direction.
In a manner in accordance with the present invention it was found that wear is reduced by the inclusion of carbon fibers in the mentioned percental amount in the slide layer, and because the carbon fibers are provided with an orientation in a preferred direction, which is selected in such a way that it extends transversely with regard to the defined relative movement of the sliding partners. The sliding partners are the bushing and the piston, or respectively the cylinder wall, in the case of an application in shock absorbers, or the bushing and the shaft, or respectively pinion, which is rotatable therein, in the case of an application of the bushing in a pivot bearing.
If an orientation of the carbon fibers essentially in or with respect to a preferred direction has been referred to above, it is understood that the carbon fibers need not necessarily be oriented parallel in relation to each other in the strictest sense, nor that each individual fiber needs to extend in this preferred direction. Instead, this means that the plurality of the fibers have undergone an orientation in this preferred direction. Therefore more than 50% of all fibers are oriented in such a way that they form an angle of less than 30xc2x0 with the preferred direction. In a very particularly preferred manner, the carbon fibers are oriented even more completely, or respectively more extensively, in the preferred direction. It was determined that it is possible to reduce wear in a bushing produced in accordance with the present invention in comparison with the slide bearing bushings mentioned at the outset, if the subsequent friction partners extend transversely to the extension of the carbon fibers, i.e. in this preferred direction.
The carbon fibers preferably have a length of 50 to 300 xcexcm and a thickness of 5 to 20 xcexcm. In a particularly advantageous manner the length is 50 to 200 xcexcm and the thickness 8 to 15 xcexcm. It has been shown that with carbon fibers of these dimensions it is possible to achieve a very extensive orientation of the carbon fibers, which constitutes the preferred direction. The tribologic properties of the composite slide bearing material achieved in this way are particularly good.
With a preferred embodiment of the present invention, the slide layer contains a carbon fiber proportion of 1.5 to 2.5% by volume, as well as a proportion of a metallic filler, preferably lead, of 18 to 22% by volume.
In a preferred manner the preferred direction of the carbon fibers is selected to be such that it extends in the circumferential direction of the slide bearing bushing, if the slide bearing bushing is intended to be used in a shock absorber. With this application the bushing has either been pressed into the cylinder and is used as the sliding partner for a reciprocating piston, or it can be displaced with the piston and is therefore provided with a radial outer slide layer and moves, together with the piston, in the axial direction against an inner cylinder surface. Although the above use of the slide bearing bushing of the present invention is particularly preferred, other uses of the slide bearing bushing are also conceivable.
The preferred direction of the carbon fibers is selected to be parallel with the longitudinal bushing axis, if the bushing is intended to be used in a pivot bearing.
Further characteristics, details and advantages of the present invention ensue from the illustration of sections through the composite material of a slide bearing bushing in accordance with the present invention and the description of a comparing measurement of the coefficient of friction.